The glistening shell is found randomly as a reward from playing Fishing Trawler. It grants an extra 60% Fishing experience during the next trawler game by using a Listen option on the shell. This yields the message: You listen to the ocean and hear whispers of greater success in your next Fishing Trawler Game. and causes the player's character to perform an animation. The effect is then applied and the shell disappears.
The boosts cannot be stacked, and players can only have one shell in their inventory at a time. The shell is an uncommon reward, but it's significantly more common than its "group" equivalent, the Shimmering shell.